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Re: [Xen-devel] [PATCH] Domain creation 4MB memory allocation

To: Jan Beulich <jbeulich@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] [PATCH] Domain creation 4MB memory allocation
From: Michal Novotny <minovotn@xxxxxxxxxx>
Date: Wed, 01 Apr 2009 11:48:33 +0200
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Jan Beulich wrote:
Michal Novotny <minovotn@xxxxxxxxxx> 01.04.09 09:49 >>>
I've found some problems when running 2 guests with and then trying to install a new machine (no matter whether using virt-manager or virt-install). I am attaching a patch that fixes this problem.

I was working on 8G box and I have done those things:
1. xm create PVguest maxmem=2500 memory=2500
2. xm create FVguest maxmem=2500 memory=2500
3. virt-install -l http://path/to/install/ -r 2500 -v --nodisks -n someGuestName

Before applying this patch it returned "Cannot allocate memory" error but after applying my patch it created a new domain and started installation well but it's reproducible sometimes and not everytime.

This is just curing (actually not really curing, just reducing the likelihood of
its occurrence) a symptom, rather than the root cause: What if 4Mb turn
out to be insufficient for someone else on some other machine? In order
to guarantee hvm guest creation to be possible (at least in the case
where shadow paging is to be used - not sure about NPT/EPT's
requirements), there has to be a way to guarantee some small amount
of contiguous memory being available in Xen. Ballooning cannot guarantee
this, and after a longer period of up-time with many guests starting and
dying even using dom0_mem= on the Xen command line doesn't help due
to fragmentation being unavoidable.


Well, in fact setting it to 4MiB is no guarantee at all and this information was done only by testing. Anyway the test case written above is the problem I run into and this solved this issue. 2MiB was surely not enough in my case and therefore I needed to patch it. Any other ideas how to make it running fine? I've tested it on some other configurations/platforms and it was working with no problems so 2MiB were really not enough.


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